FIG. 7 is a schematic view showing an overall concept of a magnetic bearing apparatus which is of a conventional five axis control type.
In the conventional magnetic bearing apparatus, a pair of right and left radial bearings 2 and 3 for a rotor 1 are disposed and a thrust bearing 4 is disposed on the right side. The radial bearing 2 is structured so that stationary electromagnets 2A and 2B radially attract a cylindrical rotor ferric core 1A mounted on the rotor 1 and magnetically bear the rotor 1 in the radial direction at its center. Also, the radial bearing 3 is structured so that stationary electromagnets 3A and 3B radially attract a cylindrical rotor ferric core 1B mounted on the rotor 1 and magnetically bear the rotor 1 in the radial direction at its center. Furthermore, the thrust bearing 4 is structured so as to magnetically receive the rotor 1 in the axial direction under the condition that a circular rotary ferric core 5 is attracted by electromagnets 6 on the right and left sides.
A radial shift sensor 7 for detecting a shift of the rotor 1 in a direction of an X-axis (X1) at a portion of the radial bearing 2 and a radial shift sensor 8 for detecting a shift of the rotor 1 in a direction of a Y-axis (Y1) at the portion of the radial bearing 2 are disposed in the portion of the radial bearing 2. Also, a radial shift sensor 9 for detecting a shift of the rotor 1 in a direction of an X-axis (X2) at a portion of the radial bearing 3 and a radial shift sensor 10 for detecting a shift of the rotor 1 in a direction of a Y-axis (Y2) at the portion of the radial bearing 3 are disposed in the portion of the radial bearing 3. Furthermore, an axial direction shift sensor 11 for detecting a shift of the rotor 1 in a Z-axis direction is disposed at a position facing an end face of the rotor 1.
FIG. 8 shows a control system for the conventional magnetic bearing apparatus shown in FIG. 7.
The control system for the magnetic bearing apparatus is provided with a microprocessor 13 for performing a calculation process, to be described later, for controlling the magnetically floated position of the rotor 1 to the centers or the like of the magnetic bearings 2 and 3. A/D (analog/digital) convertors 14 to 18 are connected on an input side of the microprocessor 13. The axial direction shift sensor 11 and radial direction shift sensors 7, 9, 8 and 10 are connected to the A/D convertors 14 to 18, respectively. Also, D/A (digital/analog) convertors 19 to 23 are connected on an output side of the microprocessor 13. The electromagnets 6, 2A, 3A, 2B and 3B are connected to the D/A convertors 19 to 23, respectively.
The operation of the conventional magnetic bearing apparatus having such an arrangement will now be described with reference to FIGS. 10 and 11.
In this case, after the rotor 1 has been magnetically lifted by the radial bearings 2 and 3 and the thrust bearing 4, when an electric power is supplied to a coil of a motor stator (not shown), the rotor 1 is brought into a rotative condition. Then, in accordance with the commands of the microprocessor 13, the A/D conversions of the detected shift signal pz of the axial direction shift sensor 11 concerning the Z-axis direction of the rotor 1 and the detected shift signals px1, px2, py1 and py2 of the radial direction shift sensors 7, 9, 8 and 10 concerning the Y-axis and X-axis of the rotor 1 are started by the A/D convertors 14 to 18 simultaneously all in the five axes (step 1). When this A/D conversion has been completed, the converted signals corresponding to the five axes are orderly entered into the microprocessor 13 (step 2), and the microprocessor 13 seeks the current command value iz of the electromagnet 6 concerning the control in the Z-axis, and the respective current command values px1, px2, py1 and py2 of the electromagnets 2A, 3A, 2B and 3B concerning the control of the X-axis and Y-axis (step 3) by a given calculation process.
When the calculation process has been completed in the microprocessor 13, the respective calculated current command values iz, px1, px2, py1, and py2 are simultaneously fed to the associated D/A convertors 19 to 23 and are D/A converted (step 4). The magnetic forces of the respective electromagnets 6, 2A, 3A, 2B and 3B are controlled so that the rotor 1 is located in a target position.
Next, an example of the calculation process of the microprocessor 13 is shown in FIG. 9.
Considering the momentum equation of the rotor 1 which is to be controlled in the conventional five axis control type magnetic bearing apparatus, it seems that the Z-axis is independent of X-axis and Y-axis, respectively. However, it is impossible that to consider that the X-axis and the Y-axis are independent of each other because of the mutual interference. Accordingly, in the calculation process of the microprocessor 13, the cross feedback is attained between the X-axis and the Y-axis. At the same time, the process is carried out independently of the one input/one output system 24 for processing the calculation in the Z-axis and the four input/four output system 25 for processing the calculation in the X- and Y-axes.
Namely, the shift signal pz that has been A/D converted in the A/D convertor 14 is subjected to a proportional differentiating/integrating calculation process in a PID (proportional integrating/differentiating) circuit 131, so that the current command value iz of the electromagnet 6 may be sought.
On the other hand, the shift signals px1 and px2 that have been A/D converted in the A/D convertors 15 and 16 are fed to the adder 132 and the subtracter 133. The output signal of the adder 132 is fed to the PID circuit 134, and the output signal of the subtracter 131 is fed to the PID circuit 135. The output signal of the PID circuit 134 is fed to the adder 136 and the subtracter 137, and the output signal of the PID circuit 135 is fed to the adder 136 and the subtracter 137 through the adder 138. Also, the output signal of the adder 133 is fed to the subtracter 145 while being multiplied by a constant K by the compensation circuit 139.
Furthermore, the shift signals py1 and py2 that have been A/D converted in the A/D convertors 17 and 18 are fed to the subtracter 141 and the adder 142, and the output signal of the subtracter 141 is fed to the PID circuit 143, and the output signal of the adder 142 is fed to the PID circuit 144. The output signal of the PID circuit 143 is fed to the adder 146 and the subtracter 147 through the subtracter 145. The output signal of the PID circuit is fed to the adder 146 and the subtracter 147. Also, the output signal of the subtracter 141 is fed to the adder 138 while being multiplied by a constant K by the compensation circuit 140.
By the way, in the case where the control system for the magnetic bearing apparatus is composed of a microprocessor as described above, in order to enhance the control performance, as shown in FIG. 11, it is necessary to shorten a process time for signals per one cycle of the microprocessor (i.e., sampling time) as much as possible. For this reason, it is necessary to use the A/D convertors which have a short conversion time (high speed type). In the conventional technology, high speed convertors have been used for all the A/D convertors 14 to 18.
However, the shorter the conversion time of the A/D convertors and higher the speed thereof, in general, the more expensive the convertors are. Accordingly, there is a problem that the more the number of the expensive A/D convertors, the more expensive the manufacturing cost for the control system of the magnetic bearing apparatus.
Furthermore, it is necessary to consider the increase in the manufacturing cost of the magnetic bearing apparatus due not only to the A/D convertors 14 to 18 but also due to the D/A convertors 19 to 23 on the output side of the microprocessor.
The present invention overcomes the foregoing drawbacks in the conventional art.
It is an object of the present invention to provide a magnetic bearing apparatus having a control system which can be manufactured at a low manufacturing cost as compared to the conventional art.
Another object of the present invention is to provide a magnetic bearing apparatus which operates without deterioration in control performance.